This invention relates generally to electronic flyback converters and, in particular, to a method and apparatus for improving modulation resolution in systems of this type.
Flyback converters, incorporating an inductor to provide voltage boost and usually implemented in boost or buck/boost topologies, can be controlled in a linear fashion through a broad range using pulsewidth modulation. The resolution of this modulation, however, is limited by the minimum amount of variance available to the controlling pulse width. This is usually not problematic in systems employing analog control, but digital systems very often rely upon integer pulse increments of a fixed clock of inadequate frequency to provide the desired resolution. A need exists to improve modulation resolution in such systems.
The present invention is a method of greatly improving modulation resolution in a flyback converter by pulse width and position modulation of a subtractive constant current with specific timing considerations.
The invention is applicable to flyback converters implemented in boost or buck/boost topologies, including circuit configurations incorporating an inductor to provide voltage boost. A typical system of this type uses a pulsewidth-modulated (PWM) circuit to provide a control pulsewidth in response to an incoming data stream. The control pulsewidth is used to drive a switching device which, when energized, charges the inductor. At release of the pulsewidth, the switching device ceases sinking current into the inductor which then, in attempting to maintain its previous current, sources voltage to the anode of a diode which conducts this voltage into one terminal of a capacitor and load resistance. The second terminals of the capacitor and load resistor are connected to the positive supply rail (through another inductor). In operation, then, when the switching device is released the energy stored in the inductor xe2x80x9cflies backxe2x80x9d through the diode into the capacitor and load resistance.
According to this invention, a constant current sink, gated under control of the PWM controller, is used to improve modulation resolution through pulsewidth- and position-modulation of a subtractive constant current with specific timing considerations. In the preferred embodiment, the constant current sink is implemented using a transistor in conjunction with a resistor network. It will be appreciated by one of skill in the art of electronic circuit design that constant current sinks using alternative components are also possible.
When enabled, the current sinks diverts current from the capacitor receiving the energy from the inductor through the diode. This action changes the slope of the discharge of the capacitor. The activation of the current sink is preferably centered on the discharge slope, with the duration being variable as a function of demand for the subtractive modulation in accordance with the incoming data stream. Where constant modulation resolution is not required, however, position modulation of constant current pulsewidths can be used to increase the available dynamic range.